The present invention relates to an apparatus for manufacturing a metal powder by the granulation of a metal melt. The metal melt in the form of one or more vertical tap streams is caused to fall down against one or more impacting streams of gas having a high velocity and is broken down or granulated by the gas into fine droplets which rapidly solidify into a powder so that the fine structure of the droplets is maintained in the powder after solidification.
The apparatus according to the present invention can be used for the granulation of all types of metallic material, but it is primarily intended for the manufacture of steel powder having a high content of alloying materials, for example, high-speed steel, or for the manufacture of superalloy powders. The above-mentioned alloys are quite difficult to manufacture by conventional casting methods due to the separation of the alloying materials, formation of segregation, deterioration of the structure by grain growth, etc., which occur upon a slow solidification of large castings.
By granulation into a powder by a jet of gas, however, rapid cooling of formed droplets can be obtained such that the temperature interval is rapidly passed where grain growth and other unfavorable structural changes take place. It is therefore possible, by hot isostatic pressing of the formed powder, to manufacture bodies in which the favorable structure in the formed powder is maintained. The following requirements regarding the properties of the powder should, if possible, be fulfilled:
(a) The powder should have a low content of impurities, above all a low oxygen content; PA1 (b) The powder should have a spherical particulate form essentially without blisters or hollows; PA1 (c) The powder should have a size distribution suitable with regard to the hot pressing; and PA1 (d) The powder should have a fine microstructure.
With regard to the requirement that the powder have a low content of impurities, a pure gas with a low oxygen content or a low content of other detrimental substances must be used. Pure nitrogen gas or a pure inert gas are used for manufacturing high-speed steel powder and superalloy powder, respectively.
Facilities for the granulation of a molten metal are expensive, both in terms of initial investment and in energy consumption. The resultant high costs for manufacturing powder have to date restricted the use of hot isostatic pressing to the manufacture of bodies from powders either formed of expensive alloys (which can be manufactured with great difficulty by conventional melting processes) or which are impossible to manufacture at all.